Glaciers and ice sheets occupy a cross-cutting role in the Earth system: as reservoirs of fresh water, glaciers nourish ecosystems and agriculture; sea level varies based on the volume of these reservoirs, a fact which has diverse implications including the alteration of human migration patterns, the modification of geological and geochemical archives, and the revision of coastal hazards; glaciers sculpt some of Earth's most extreme landscapes; and past global-scale glaciations may have been fundamentally implicated in the evolution of life.